The invention concerns an aluminum alloy of AlCu basis containing cadmium and manganese as further additions, and concerns too the use of the said alloy.
Aluminum alloys of the AlCu type belong to the group of so called high strength aluminum alloys. Their main application is in aircraft construction. It has been known for some decades now that the addition of magnesium to AlCu alloys accelerates both natural aging and artificial aging and is therefore a method of improving the age hardening of such alloys. The magnesium addition also causes the strength level which can be reached by artificial or natural aging to be raised considerably. Furthermore, on artificial age hardening AlCu alloys containing Mg, instead of forming the .theta." and .theta.' phases of the binary alloy, the more thermally stable magnesium containing intermediate phases S" and S' are formed; this results in higher strength at elevated temperatures.
However, artificially aged magnesium containing AlCu alloys exhibit very poor toughness properties and pronounced susceptibility to intercrystalline corrosion and stress corrosion. A further disadvantage of AlCuMg alloys is their very poor formability, in particular the extrudability. This makes it impossible to manufacture complicated extruded sections.
Attempts have been made to replace magnesium by cadmium. For example from the patents CH-Pat. No. 318 523 and GB-Pat. No. 709 527 AlCuCd alloys are known with additions of magnesium, tin, manganese, iron, silicon, and further impurities, and additions of zinc, nickel, chromium, molybdenum, zirconium, beryllium, cerium, boron, titanium, silver and lead.
It is already known from the above mentioned patent CH-Pat. No. 318 523 that alloys of the AlCuCd type exhibit certain advantages over alloys of the AlCuMg type viz,
(a) they can, for example, be hot worked by rolling, drawing or forging, without forming cracks. PA1 (b) The deformation can be carried out at high speed. PA1 (c) In the worked condition the AlCuCd alloys exhibit less anisotropy in their properties than alloys of the AlCuMg type. PA1 (a) high strength, PA1 (b) good formability, PA1 (c) good corrosion properties, in particular resistance to stress corrosion and intercrystalline corrosion, PA1 4.0 to 5.0% copper, preferably 4.4 to 4.7% PA1 0.1 to 0.2% cadmium, preferably 0.13 to 0.17% PA1 0.2 to 1.0% manganese, preferably 0.4 to 0.7% PA1 zirconium 0.1 to 0.4%, preferably 0.17 to 0.22% PA1 vanadium 0.1 to 0.2%, preferably 0.13 to 0.17% PA1 The alloy can also be extruded into complicated sections. PA1 The alloy allows extrusion welds to be formed i.e. it is also suitable for the manufacture of tubes. PA1 The alloy exhibits extremely good hot strength. PA1 The alloy can be water quenched from the extrusion temperature and then, at a later point in time e.g. after machining, can be age hardened.
From the literature mentioned one learns in general--even if to some extent by way of implication--that because of their mechanical properties such as:
AlCuCd alloys must be extremely good as alloys for construction purposes. In spite of this knowledge, AlCuCd alloys have, up to now, not been able to find use in practice as they did not adequately provide the solutions to the problems encountered in practice. This is due in particular to the fact that the combination of the three properties viz, strength, toughness and corrosion resistance was not, or only insufficiently, considered. Both of the above mentioned patents encompass such a variety of possible combinations of alloying elements and also such wide concentration limits that--except for the above general information--they do not teach the expert anything of any practical use. The inventors therefore set themselves the task of developing an alloy of the AlCuCd type which satisfies the highest demands made on construction alloys in terms of strength, toughness and corrosion resistance.